1. Field of the Invention
The present invention relates to a storage and retrieval system and method for cargo containers and, more particularly, to the use and adaptation of this system in the conversion of existing vessels or storage facilities as well as in the construction of new vessels or storage facilities.
2. Description of the Related Art
It is to be understood that according to this disclosure a "container" is a standardized packing case for cargo in which goods can be safely stored and/or transported, by road, rail or sea. In the cargo container industry a size of the container has been standardized by the International Organization for Standardization. The container size that is used as a unit of measure is the 20-foot length container and, thus, the 20-foot equivalent unit (TEU) is the basic unit of measure used when making calculations of, for example, storage capacity. There are also standardized containers with lengths of 10, 30, 40 and 45 feet. The containers have also been standardized to a width of 8 feet and a height of 8 or 8.5 feet.
Economies of scale have led to the design and construction of containerships large enough to carry at least 6,000 TEU's. Thus, containerships and corresponding containers have evolved along with a wide array of handling equipment and apparatus. For example, container bridges, straddle carriers, stacking cranes, gantry cranes, toplift trucks, sidelift trucks, forklift trucks, vehicles utilized for moving the containers, and computer and tracking systems are just some of the adjuncts that have developed to service loading and unloading of containers and containerships. A containership can be loaded or discharged, for example, in a few days in spite of the fact that a containership carries more than twice as much cargo as a conventional vessel of the same size. Containerships are thus able to make more voyages and generate much higher profits in the course of a year than conventional vessels can. The manageability of these uniformly structured containers has encouraged production and commerce, promoted consumerism, and produced modern and diverse means of transportation.
A loaded containership typically has on its deck and deep into its cargo hold a thousand or more standardized containers stacked on top of each other. Containerships typically have extremely wide hatchways which are essential so that the containers can be lowered into the cargo hold across the width of the ship, maneuvered vertically into cells and stacked on top of one another. The cells guide the containers into the hold and prevent the containers from shifting or tipping over at sea. However, the size of the hatchways also affects the ship's strength across the ship's deck area. Thus, in order to compensate for weaknesses in the ship's hull, girders are typically placed into the containership over bulkheads on the sides of the ship, in a girder construction, to add rigidity.
A beam of a containership or, in other words, a greatest width of a containership, its height from the water line to the uppermost deck in a loaded ship, as well as a ship's center of gravity are typically managed so that when the ship is correctly loaded, the ship has stability in heavy seas. In addition, a containership typically has a ballast system with a large double bottom, ballast side tanks and extensive high-performance pumps to ensure that the ship's balance or trim is maintained according to the cargo being carried. For example, the ship's inclination to heel to one side can be countered by flooding or draining the side ballast tanks or "counter trimming" as known to those of skill in the art. A heel compensating system typically pumps ballast water from one side tank to another side tank of the containership during loading and unloading of the containership so as to maintain trim of the containership. One of the reasons for this is because it is only possible to load or unload the containers from the cells of the ship when the containership is in an upright position. In addition, when a ship has an even keel, the ship has the least draft possible, which is an advantage when the ship is sailing in shallow waters.
These containerships typically also have a spreader that is used to lift up, carry and lower the containers into and out of the cargo hold of the containership. The spreader is equipped with four small twist lock receptacles that mate with four corresponding twist locks on the container and can be used to grip the top corners of the container, and, for example, by remote control, be interlocked with the container to accomplish the loading and unloading of the containers. Just as the container size is standardized, the twist locks and receptacles are typically standardized so that the containers can be handled by every port in the world equipped to do so.
The capital investment required for the construction of vessels of this size, however, has led to consolidation among shipping companies, and competition has led to the loss of lower profit feeder links and shuttles between smaller shipping ports. In addition to market pressures, shallow water ports cannot be serviced by these larger vessels, nor is it economically or logistically feasible to reestablish feeder links by replacing obsolete vessels with small, but much more expensive containerships. Because current containerships or smaller vessels do not permit convenient, instantaneous and systematic retrieval of shipboard containers, most of the world's smaller or shallow water ports are unable to reach their full potential.